The mechanisms of polymerisation

Two mechanisms are known to operate in the process of polymerisation addition polymerisation and condensation polymerisation. Each results in the formation of a certain type of polymeric structure. 

In addition polymerisation the monomers are simply added together. This process involves the breaking down of a double covalent bond between two carbon atoms and the subsequent redistribution of these bonds as single bonds between a whole series of carbon atoms. In the formation of polyethylene (polythene) from ethylene this occurs as shown below. 

In contrast, polymerisation by condensation involves a reaction between active groups, of which there must be at least ty/o in each molecule. During the polymerisation process simple, non-polymerisable, molecules are condensed out as byproducts. In the case of the formation of nylon 66 from diamine and dicarboxylic acid, for example, water is the small molecule that is eliminated or condensed, as shown below  

Addition polymers can be produced by opening a double bond by the application of sufficiently high temperatures and pressures to the material, but most processes involve the addition of a small amount of a very reactive substance, called an initiator, to the 

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In general the mechanism of polymerisation for thiophene appears to be similar to that of pyrrole (Section 4.11.2), occurring via a radical coupling mechanism [423] giving mainly a-a linkages [293,400,405], and involves oligomer as well as monomer radicals, with evidence to suggest that the polymerisation reaction occurs at a lower... 

The combination of a number of these techniques often provides complementary information on end group structure and hence the mechanism of polymerisation. The synergy between MALDI-TOF MS and NMR spectroscopy is particularly powerful, with MS/MS providing additional information where necessary. MALDI-TOF MS provides information on individual oligomers,... 

From the foregoing discussion it appears that the mechanism of polymerisation of cyclic formals may differ considerably from that by which cyclic ethers polymerise,... 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

Tertiary amines were amongst the first initiators of NCA polymerisation which had been described in the literature and it seems that the polymerisation of all the known NCA s may be accomplished by their action. Wessely (77) reported in 1925 that glycine and phenyl alanine NCA s are readily polymerised in pyridine at ambient temperatures, and in the following paper (72) he reported a similar polymerisation of sarcosine NCA. The polypeptides produced by this initiator apparently formed cyclic polymers since no terminal end groups could be detected 41). It is significant that appreciable quantities (a few %) ot 3-acetic-hydantoin derivatives were found in the polymers formed from glycine and phenyl alanine NCA s but none was detected in the polymerised sarcosine NCA (72). This evidence suggests that the mechanisms of polymerisation initiated by aprotic bases may be different for the non-N-substituted NCA and the N-substituded anhydrides. 

Several reports describe the mechanism of polymerisations using cerium compounds in combination with alcohols77,78, diols79,80 and polyols81. Only in a limited number of kinetic studies was the concentration of cerium ions measured during the polymerisation reaction. Mohanty et alm, Rout et al.m, Jayakrishnan et al.65 and Gupta and Behani.67 followed the reduction of Ce(IV) ions titrimetrically. 

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

Interesting evidence supporting the mechanism of polymerisation of acetylenes via carbene species is provided by the block and random copolymerisation of acetylenic monomers with cycloolefins. For instance, block copolymers of acetylene and cyclopentene with the WC —AlEtCT catalyst [41] and block copolymers of acetylene and norbornene with the (MeA. Oj2W(=NAr)= CHMe3 catalyst [42] have been obtained moreover, random copolymers of phenylacetylene and norbornene with the WC16 catalyst have also been obtained [149, 150],... 

It was finally stated that the formation of active species from two molecules of acid was supposed to be a determining-step in the mechanism of polymerisation (sic). 

If the membranes cannot come closer together than a certain minimum distance, initiation may be seen as a way of extending the nascent chains across this gap, so that they come within range of the mechanism of polymerisation. 

Finally, artificial polysaccharides produced via in vitro enzymatic synthesis [228] are new biomaterials with defined structures which either mimic natural polysaccharides or have nonnatural structures and functionalities. Polysaccharides are obtained by the enzymatic polymerisation of simple glycosyl donors via repetitive condensation. This approach not only provides a powerful methodology to produce polysaccharides with defined structures and morphologies as novel biomaterials, but also represents an available tool to analyse the mechanisms of polymerisation and packing in order to acquire high-order molecular assemblies. 

All these observations can be rationalised by assuming the mechanism of polymerisation to be similar to free-radical addition of thiols to olefins as outlined next. 

Kojima and co-workers [52] identified the major thermal decomposition products of plasma polymerised polypyrrole as nitriles with less than four carbons and alkyl pyrroles. Evolution of only monosubstituted alkyl pyrroles, such as 2-methylpyrrole and 2-ethylpyrrole, suggests that polypyrroles consist of monosubstituted pyrrole rings. This is also supported by the result that the IR spectrum of polypyrroles differs from that of the electrochemically polymerised pyrrole, which consists of disubstituted pyrrole rings. Evolution of linear nitriles shows evidence that a polypyrrole molecule has the main chain containing nitrogen atoms. The mechanism of polymerisation of pyrrole in the discharge is considered to be similar to that of aromatic hydrocarbons, which mechanism involves a process of production of acetylene. 

The chain length distribution of a polymer has a great influence on the mechanical properties and the processability. From the mechanism of polymerisation, it should be possible to calculate the chain length distribution of the formed polymer chains. 

NBR is made as an emnlsion with a free radical initiator. Therefore, the mechanisms of polymerisation and branch (gel) formation are very similar to those of E-SBR. Polymers are made with an acrylonitrile (AN) content of, for example, 28, 33 or 40 weight percent, depending upon the required oil resistance. The copolymerisation is described as random but the reactivity ratio is such that it is biased towards an alternating sequence. Becanse of this and because AN is the minor component, it is used up early and polymer formation at a later stage of polymerisation becomes butadiene rich [7]. In order to have a uniform composition among polymer chains, the monomer mixture of the prescribed ratio is metered in with the progress of polymerisation. 

This rubber is made by the emulsion polymerisation with a free radical initiator. Therefore, the mechanisms of polymerisation and the long-branch (gel) formation are the same as those of SBR and NBR. However, there are some significant differences in the nature of the chain ... 

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